An electromagnetic lens is designed to focus the electron beam by using a suitable shaped magnetic field. The lens is a symmetric electromagnet consisting of wire coil, magnetic iron yoke and iron pole pieces. A strong magnetic field is produced by passing a current through the windings. This field acts as a convex lens; converge off axis rays back to focus. Like an optical lens, a magnetic lens is characterized by focal length and can be altered by changing the strength of the current.

(EMR) Electromagnetic radiation consists of an electric and a magnetic field component. All EMR travels in a vacuum at the speed of light. EMR is classified related to the frequency//length of the wave.
An EM wave consists of discrete packets of energy, named photons (quantization). The energy of the photons depends on the frequency of the wave. Planck-Einstein equation:
E = h * f
E (energy); h (Planck's constant); f (frequency)
EMR types include in order of increasing frequency//decreasing wavelength: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays. EMR contains energy and momentum, which may be imparted when it interacts with matter.

See Gamma Radiation.

The near field is one of the boundary regions to classify characteristics of electromagnetic fields as a function of distance from the radiating source. The variation of the electromagnetic wave is usually more rapid in the near field than in the far field.

(RF) Radio frequency refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna.

X-rays are a part of the electromagnetic spectrum. X-rays and gamma rays are differentiated on the origin of the radiation, not on the wavelength, frequency, or the energy. X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus. X-rays have wavelengths in the range of about 1 nanometer (nm) to 10 picometer (pm), frequencies in the range of 10^-16 to 10^-20 Hertz (Hz) and photon energies between 0.12 and 120 kilo electron Volt (keV). The energy of rays increase with decreased wavelengths. X-rays with energies between 10 keV and a few hundred keV are considered hard X-rays. The cutoff between soft or hard X-rays is around a wavelength of 100 pm.
Because of their short wavelength, X-rays interact little with matter and pass through a wide range of materials. These interactions occur as absorption or scattering;; primary are the photoelectric effect, Compton scattering and, for ultrahigh photon energies of above 1.022 mega electron Volt (MeV), pair production.
X-rays are produced when high energy electrons struck a metal target. The kinetic energy of the electrons is transformed into electromagnetic energy when the electrons are abruptly decelerated (also called bremsstrahlung radiation, or braking radiation) similar to the deceleration of the circulating electron beam in a synchrotron particle accelerator. Another type of rays is produced by the inner, more tightly bound electrons in atoms;; frequently occurring in decay of radionuclides (characteristic radiation, gamma ray, beta ray). The energy of an X-ray is equivalent to the difference in energy of the initial and final atomic state minus the binding energy of the electron.
Wilhelm Conrad Roentgen discovered this type of rays (also called Roentgen-rays) in 1895 and realized that X-rays penetrate soft tissue but are absorbed by bones, which provides the possibility to image anatomic structures; the first type of diagnostic imaging was established. Radiographic images are based on this difference in attenuation for tissue and organs of different density. Today ionizing radiation is widely used in medicine in the field of radiology.

See also Exposure Factors, X-Ray Tube, and X-Ray Spectrum.